1. Field of the Invention
The present invention relates to a stainless steel boiler tube having an anticorrosive coating, which is resistive to the attack by high-temperature, high-pressure steam, formed on at least an inner surface of the tube, and a method for manufacturing the same.
2. Description of the Prior Art
Recently, there has been a rapid trend of the elevation of the temperature and the increase of the pressure and the capacity of a boiler for a power station and hence the elevation of the temperature at the center of the boiler. Thus, a higher corrosion resistance than in the prior art boiler has been required. For this reason, a boiler tube of stainless steel has been used in high-temperature section of the boiler.
Even with such a stainless steel boiler tube, the corrosion occurs on the inner surface of the tube when high-temperature, high-pressure water or steam is passed therethrough. The resulting scale will be exfoliated when the thickness thereof exceeds a predetermined value. The inventors of the present invention have found that such exfoliating would not occur during the contact with the high-temperature, high-pressure steam but would occur when the temperature of the boiler tube was lowered to a room temperature to about 200.degree.c while the operation of the boiler was stopped for the inspection of a turbine. When the operation of the boiler is stopped by a reason not associated with the inspection of the boiler, it is usually re-started without inspection. On the other hand, the accumulation of the exfoliated scales cannot be detected unless the boiler tube is cut. If the boiler is re-started while the exfoliated scales are accumulated, the tube will be overheated, which will lead to an accident, i.e. the tube bursts.
As an approach to overcome the above difficulty, it has been suggested to air-oxidize the inner surface of the tube to form an oxide coating on the inner surface to suppress the corrosion. According to that approach, ferrous sesquioxide which is referred to as red with rust, magnetite (Fe.sub.3 O.sub.4) which is referred to as black with rust, ferrous suboxide and composite oxide of chromium, nickel and iron are formed in layers in the above order in the direction of the depth. Of those layers, only the composite oxide layer is effective to prevent the corrosion. However, in the above approach, the formation rate of the effective anticorrosive coating is low, and the adherence to the metal surface is poor and a sufficient anticorrosion effect cannot be provided because of the thin composite oxide layer. When water sufficient to meet the requirement for the operation of the boiler is passed through the stainless steel boiler tube having the above oxide coating, the rust primarily consisting of Fe.sub.3 O.sub.4 grows and eventually the rust will be peeled. Thus, as in the previous description, the scales accumulate at the bottom of the boiler tube, which leads to a break in the boiler tube. In order to prevent the break, it has been required in the past to cut portions of the boiler tube to mechanically remove the rusts accumulated at the bottom of the boiler tube or to provide a valve seat on the tube from which chemicals are injected to chemically remove the accumulated rust. However, both removal methods require must cost and much time. In addition, the mechanical removal method cannot completely remove the scales while the chemical removal method remove the effective anticorrosive coating as well.